Low temperature co-fired ceramics (LTCC) is one of the core technologies for passive integration and package-interconnection, in which low-temperature fired ceramic powder is made into a green tape, then a required circuit pattern is formed on the green tape by drilling, micropore grouting, electrode printing, etc., and various passive components (such as capacitors, resistors, filters, couplers, etc.) are embedded in a multilayer ceramic substrate, laminated together, and sintered below 950° C., to give a three-dimensional high-density circuit or a three-dimensional circuit substrate with built-in passive components, and IC and active devices are mounted on the surface of the three-dimensional circuit substrate to give a passive/active integrated functional module which is especially suitable for a subassembly for high-frequency communications. Because of its excellent electrical, mechanical, thermal, and process characteristics, LTCC has become a core technology for miniaturization, integration, and modularization of electronic components, and been widely used in the areas of aviation, aerospace, military, automotive electronics, wireless communications, etc.
With the development of next-generation mobile communications, satellite navigation and positioning systems, intelligent networks, and unmanned aerial vehicles, mass-data high-speed wireless transmission of information is the development trend of the future. Therefore, the new generation of LTCC passive components must meet the requirements of high-frequency, broad-band, and low-loss. LTCC materials are the basis of the application of LTCC technology. In order to develop a new generation of LTCC passive components, an LTCC microwave dielectric material which can be used at a frequency above 30 GHz or even above 100 GHz is needed.
Glass ceramics, and glass-ceramics are the most typical composition systems of LTCC material. It is often necessary to introduce a glass phase to achieve low-temperature sintering; however, the disordered structure of the glass phase leads to a great intrinsic loss of the LTCC material. Therefore, material researchers have been exploring the problem of how to maintain good microwave dielectric properties while achieving low-temperature sintering.
Many researchers hope to develop glass-phase-free LTCC microwave dielectric materials based on crystalline compounds of low melting points, such as molybdate (CN201010192027), tungstate (Journal of the American Ceramic Society, V95, No. 1, p. 318-23, 2012), tellurite (Journal of the European Ceramic Society, V21, p. 1735-1738, 2001), phosphate (Journal of the European Ceramic Society, V33, No. 1, p. 87-93, 2013), etc. Although these single-phase crystalline compounds exhibit excellent microwave dielectric and sintering properties, because the properties of the materials are determined by their intrinsic characteristics, it is difficult for the materials to match other materials to form a co-adapted material system, and thus there is still a long way to go before practical application can be made thereof.
U.S. Pat. No. 5,258,335 and related patents provide a high performance glass ceramic based LTCC microwave dielectric material with a dielectric constant of less than 7.8 and a dielectric loss of less than 10−3 which is obtained by firing CaO—B2O3—SiO2 glass at around 900° C. for crystallization, and a Ferro A6M material developed therefrom can be used at a frequency range of 10 GHz to 100 GHz. However, in the methods of these patents, the material is crystallized by controlling the sintering temperature. Therefore, the properties of the material, which are highly related to the type and content of the crystalline phase, are very sensitive to the sintering process parameters.
The method for preparing a glass ceramic comprises: melting raw materials at a high temperature, rapidly cooling the melt by water quenching or the like to get a glass phase, crystallizing the glass phase by heat treatment at a temperature to form compounds having fine crystal grains, thus obtaining a glass ceramic with microcrystal phases dispersed in the glass phase wherein the type and content of the microcrystal phase compounds have a decisive impact on the performance of the glass ceramic. A glass ceramic is generally composed of elements that can form crystalline phase compounds. Therefore, it is possible to obtain a crystalline phase by direct solid-state synthesis. However, there has not been a report of obtaining an excellent performance LTCC ceramic material by direct solid-state synthesis.